Frequency modulation receiver



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Oct. 3l, 1944. R, WEAGANT 2,361,669

FREQUENCY MODULATION RECEIVER Filed may 2e, 1942 IZ lNvENTOR for //EAqA/VT ATTORN EY Patentedy Oct. :'31, '1944 UNITED' STATE s* PATENTA oFFlcr-zassmse .FREQUENCY MonULa'r'IoN anonima' Roy A. Weaga'nt, Douglaston, N. Y., assigner-.to Radio Corporation of America, a corporation of Delaware Application May ze, .1942, serial No. 444.506'

lMy present invention' relates generally to a method of, and means for, Vminimizing amplitude modulation effects in receivers of angular veloc- .ity-modulated carrier waves, and more particularly to areceiver of frequency modulated car. rier waves embodying a novel and simpliiiedV formi I of amplitude modulation elimination means.

One of the main objects of my-invention is to provide a 4receiver of angular velocity-modulated carrier waves which embodies a carrier wave de generative circuit prior tothe demodulator network; the degenerative circuit functioning Lto insure a substantiallyat-top response curve at the network feeding the demodulator.

Another important object of my invention is to provide in combination with a demodulator of angular velocity-modulated carrier waves a stage'of amplification of suchLwaves; the amplification stage including a degenerative, circuit arrangement which insures a substantially ilattop rresponse. curvethereby preventing amplitude modulation eiectsfr'om arising in the waves appliedv to the demodulator.

Another. 'object of this invention. isto provide i a novel method of receiving frequency modulated carrier waves (FM) without the use of an ampli-V tudelmodulation limiter per se; and at least one amplier prior to the receiver demodulator embodying. a simple circuit device for minimizing amplitudemodulation effects.

Still other (objects of the .invention are to im- FM receiver substantially free of amplitude modulation effects whichv commonly arisev in. such4 receivers. The novel features which I believe to .hechara'cteristic of m57 invention'are set forth with par-A ticularity in' the appended claims; the invention itself, however, as to-both its organization and 'method ofoperation will best be understood by Vreference to the following description taken in connection' with the 'drawing in which I have indicated diagrammatically acircuit organizaf' tion wherebvmy invention may be carried into eifect.\

In theidrawing: Fig. Ischematically shows a receiver ing the meth'od of the invention,

Fig. 2a illustratesithe ideal response curve of the radio frequency amplifier network,

Fig.- 2b. graphically illustrates two types of unembodycorrected intermediate frequency (I. F.) pass Aband curves commonly encountered in FM receivers,

f1 Claims. (Cl. 250g-20) l Fig. 2c shows the effect of lusing the present invention,

tion, Y

Fig. 4 illustrates a 'modified form of the i1 1 vention. y

Referring now to the accompanying drawing, wherein like reference characters in the different iigures designate similar circuit elements, the

.schematic networks of an FM receiver as shown in Fig. 1 generally comprise a signal collector l which feeds one or more stages of radio frequency amplification 2. The collector may be of any desired type, a dipole being shown merely .by way of example. The assigned FM frequency modulation) broadcast band covers a range of 42 .'to 50 megacycles (mc.), and each FM channel has a width of 200 kilocycles (kc). At'the FM transmitter the station carrier is actually de- Vplitude modulation effects. This is obviously prove generally the, simplicity 'and efficiency of FM receivers, andmoreespecially to provide an fas necessary because 'it is the' function of the FM demodulator to derive the modulation signals from the amplified FM waves. The introduction of amplitude modulation components in the FM waves applied to the detector results monic distortion.

The 'ampliiier 2 is given a Wide Apass band, such is illustrated lin Fig. 2a. From that gure it will be seen thatthe pass band is substantially wider than the overall carrier deviation whose 'extremities are spaced some 150 kc, apart. The

radio'freque'ncy amplifier 2 will include one or more tunable selector circuits which are, of

course, adjusted to the center frequency of the desired FM channel. By giving the radio frequency amplier a wide flat-top response characteristic, and Whose width is substantially Wider than the overall frequency deviation range of the received FM' waves, amplitude modulation effects-due to the selector circuits prior to the -rst detector 3 will not appear. The local oscillator 4 is tuned through a frequency range such las to produce local oscillations which, whenapplied to the converter stage 3.

Fig. 3 illustrates one embodiment of the invenl in harproduces the I. F. energy in the output circuit of f the latter'. The I. F. energy covers the same frequency deviation range as the radio frequency energy, but thecenter frequency has been reduced to the operating I. F. value. The latter may be chosen from a range of 2 to 18 megacycles -(mc), such as, for example, 4.3 mc.

The I. F. amplier 5 may comprise one or more stages of I. F. amplification. Each I, F. amplifier comprises, of course, avtube having a band pass input circuit and a band pass output circuit. 'I'he FM detector 6 has a discriminator input network 1 which may be of the type disclosed and claimed by S. W. Seeley in his U. S. Patent 2,121,103, granted June 21, 1938. Of course, any other type of FM detector circuit may be employed. The audio frequency output of thedetector may be amplified in one or ,more stages of audio frequency amplification, and a reproducer will follow the audio frequency amplifier network. i From a generic viewpoint I utilize degenerative carrier feedback in producing the novel method of my invention. yThe numeral 8 in Fig. 1 denotes the degenerative feedback path, and it is lt`o be understood that the I. F. energy may be degeneratively fed back from the I. F. ampliner output tog its input inl any predetermined amount `which will produce thedesirable results of this of the present method, attention is directed to Fig. 2b which shows the usual type of uncorrected I. F. pass band encountered in FM receivers of present day construction. 'I'he curve` A has av r1; will be `noted that amplitudek modulation effects introduced by either of curves A or B of Fig. 2b

are virtually eliminated by virtue of the flat-top response curve of Fig. 2c.

In Fig. 3 there is shown a desirable method of introducing I. F. degeneration in the I. F. amplifier immediately preceding the detector. It will be understood that the transformer 9 is an I. F. transformer whose primary and secondary circuits are each flxedly tuned to the center frequency of the applied FM waves, in this case the coupled to the resonant input circuit I5 of the' FM detector. Here, again, as pointed out above, the detector is of the well known type disclosed .in the aforesaid Seeley patent.` It is not/"believed necessary to describe the operati/on of the detector. Its functioning is clearly 4described in the Seeley patent, and 'it is Well known at this datey to those skilledin the art of constructing FM receivers. ,Suiilcientde'generative feedback round top, whil'e the curve B has a central de pression with spaced peaks. Either of these response curves in the I. F. networks will cause amplitude modulation effects to' appear in the energy impressed upon the discriminator input network of the demodulator. The response curves of the types of A and B arise by virtue of d if it were theoretically possible to make the re- 7 sponse 'curve of the I. F. ampller flat enough,`

many causes. In the rst place, it is diflcult to manufacture. an I. F. transformerliaving coupled primary and secondary circuits whose pass band will have a at top. In the second place, even as a practical matter the I. F. amplier components are not stable enough to insure that the amplier would remain aligned withl this characteristic. Temperature, humidity, vibration and ageing effects cause even'. the best of present day receivers to drift out of line. l y

In the past various, arrangements have been .proposed for minimizing amplitude modulation effects introduced by virtue of the failure to provide a fiat-top response curve in the I. F. amplier feeding the detector.

been inserted immediatelyprior to the demodulator. However, a limiter stage requires that the A limiter stage has is had' to the 'grid-cathode circuit of I; F. amplifier I0 to convert the undesirable response curves of Fig. 2b to the highly desirable nat-top response curves of Fig. 2c. There is fed back to the lgrid circuit of amplifier I0 FM wave voltage developed across circuit I2, and the feedback of the voltage is in a degenerative phase. Hence, it will be seen that no additional stage of limiting is required in my present method. -It is but necessary to f provide in the cathode lead of the I. F. amplier I0 a coil. I3. The complete simplicity of the present invention, as contrastedto the relatively complex devices previously used .to compensate for modulation effects, will be obvious to those skilled in theart. Of course,l the I. F. stage prior to the amplifier l0 nmay also embody a degenerative feedback coil in its cathode lead. e

In Fig. 4 there is shown amodiiied form of a i degenerative circuit. In this case the I. F. ampliner tube I0 has its cathode connected to ground through a path comprising the carrier-bypassed biasing resistor I4, adjustable tap,.20, and the portion of coil 2i between tap 20 and ground.

.'I'he coil 2| is connected in shunt with a con- -ing Such bias. f

One of 4theadvantages of utilizing degenerative FM'voltage feedback in vone or more I. F.

components required. According to my present invention suilicient I. F. degeneration is intro-V duced in the I. F. amplifier network so as to provide an I. F. response curve as shown in Fig. 2c.

amplifier stages is the fact that such factors as temperature, humidity, vibration-and ageing efbiasing resisfects do not appreciably affect a degenerativeV amplifier stage. Hence, the :dat-top resonance A curve will be maintained in spite of. the usual veffects which normally cause the I. F. amplier to drift out of alignment with the desired characteristic. It is, also, pointed out that the balanced detector which is employed reduces amplitude modulation to a degree which is suicient for many purposes. The balance in the FM demodulator network rejects noise during no-sig'nal periods but allows the noise to rise as the depth of modulation rises.A Since the modulation rises with the noise, the effect is not as annoying as though it were continuously present. By utilizing the present invention to insure a at-top response curve in the I. F. network feeding the detector the action of the balanced FM demodulator isenhanced. It is, also, of interest to note that, as employed under the present FM broadcasting channel assignments, the signal to noise ratio at the collector of the usual FM receiver is relatively high so that external noises are not an appreciable factor in the production of amplitude modulation.

While I have indicated and described several systems for carrying my invention into effect,l it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications maybe made without departing `from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a receiver of angular velocity-modulated carrier waves of the type comprising ademodulato'r of such waves preceded-by an amplifier which is tuned to the mid-band frequency of the provement which comprises means'for providing sufficient degenerative feedback of -modulated carrier wave voltage between the output and input circuits of said amplier to insure a sub-- stantially flat top-response curve for said amplier over said pass band thereby to eliminate said amplitude modulation. l p

2. In combination, in a frequency modulated carrier wave receiver, a frequencyv modulation detector, an amplifier having a-resonant input Y.

circuit and a. resonant output circuit, means .coupling said resonant output circuit'to said detector, said amplier having a pass band at least as wide as the over-all frequency deviation range of the ampliiied waves but whose response is such that amplitude modulation of the received waves occurs, and means providing sufficient degenerative feedback of the amplied modulated carrier wave energy between said resonant output circuit and said input circuit thereby to provide a. substantially flat-top response curve over said pass band wherebysaid amplitude modulation is substantially reduced.

3, In a frequency modulation receiver of the superheterodyne type which includes at least one .intermediate frequency amplifier having a nonuniform response curve such that amplitude modulation of received 'waves arises prior to a frequency modulation detector of the balanced type adapted to reject noise solely-during nosignal periods; the improvement which comprises means providing suilicient degenerative feedback of frequency modulated carrier wave voltage between the output and input circuits of said ampliiler thereby to insure a substantially at-top response curve for said ampliiier whereby the noiserejection action of the balanced detector is enhanced.

4. In combination, in a frequency modulated carrier wave receiver, a frequency. modulation 'detectorof the balanced type which has a noise rejection characteristic during no-signal periods but which allows noise to rise as the depth of 'modulation rises, an amplier having a resonant input circuit and a resonant output circuit, means coupling said resonant output circuit to said detector, said amplier having a pass band at least as wide as `the over-all frequency deviation range of the amplied waves but whose shape is such as to give rise to amplitude modulation of the waves, and means in the space current path of the amplifier providing suillcient degenerative feedback of the amplified modulated carrier wave energy between said resonant output circuit and said input circuit to provide a substantially attop response curve over said pass band whereby said noise rejection characteristic is augmented during signal periods.

' ROY A. WEAGANT. 

